Rendering a fake mirror
You can use &quot;Is reflection ray&quot; output from &quot;Light path&quot; node and plug it into the alpha input of the &quot;Principled shader&quot;.
This will work for both Eevee and Cycles. For Eevee you need to set &quot;Blend mode&quot; to alpha hashed in the material setting.

For the one that reflects in the mirror but is not visible outside of it:

For the one that is visible only outside of the mirror:

For Eevee, set both objects to alpha hashed in the material settings: